Freckles are clusters of concentrated melaninized cells which are most easily visible on people with a fair complexion. The formation of freckles is triggered by exposure to sunlight.

The exposure to UV-B radiation activates melanocytes to increase melanin production, which can cause freckles to become darker and more visible.

Freckles are predominantly found on the face, although they may appear on any skin exposed to the sun, such as arms or shoulders.  It is suggested that people whose skin tends to freckle should avoid overexposure to sun.